Magnetic denture retention devices have been developed by Dr. Barrie R. D. Gillings of the Univeristy of Malaya in Malaysia, these being described in his U.S. Pat. Nos. 4,209,905 and 4,302,189. The use of permanent magnets as denture retention devices is described in various publications by Dr. Gillings. For example, paired bar permanent magnets, embedded into the maxillary and mandibular dentures, have been used for denture retention, through the mutual repulsion of like poles of the magnets. Permanent magnets have also been used as implants in the bone of the lower jaw as retention aids for mandibular dentures containing opposite polarity magnets, so that magnetic attraction will occur between the implanted magnets and the magnets in the dentures.
Recently, a new magnetic alloy, composed of cobalt and samarium (CO.sub.5 Sm) has become available. Permanent magnets made from this alloy not only exhibit extremely high magnetic field strength, but they also possess extremely high coercivity. This coercivity is so high in cobalt/samarium magnets that the magnets can be made extremely short without the north pole tending to demagnetize the south pole. This unique property is such that cylindrical-shaped permanent magnets of a diameter, for example, of 3 millimeters, and of a length of 2 to 3 millimeters, can provide magnetic attraction forces in excess of 300 grams. This property of the cobalt/samarium magnet renders it ideal for use in the unit of the present invention.
It is widely believed that the magnetic fields produced by permanent magnets can damage tissues, when such magnets are used in patients' mouths. This belief has inhibited the use of permanent magnets for denture retention purposes in the past. However, Dr. Gillings in the publications referred to above describes magnetic type denture retention units which do not exhibit any significant external magnetic field, so as to remove any objection to the use of his devices for denture retention purposes.
The Gillings closed field magnetic retention unit comprises a pair of oppositely-poled permanent magnets, preferably of the cobalt/samarium type placed adjacent to one another in a slightly spaced relationship. A first ferromagnetic keeper is placed across one end of the two magnets in contact with the magnets to form a first magnetic path between the opposite poles of the two magnets at that end. In accordance with Dr. Gillings' teachings, the two magnets and the first keeper are embedded in the denture with the two pole faces at the opposite ends of the magnets being exposed at the surface of the denture. A second keeper of ferromagnetic material is embedded in the root in the patient's mouth which is to serve as an anchor for the denture. When the denture is in place, it is magnetically retained on the root because of the magnetic attraction of the second keeper and the permanent magnets. Moreover, the resulting assembly produces a closed magnetic circuit from the poles of the permanent magnets through the two keepers, so that there is no external magnetic field.
U.S. Pat. No. 4,431,419 which issued Feb. 14, 1984 in the name of the present inventor discloses a unit which achieves the same results as the Gillings' unit, but which may be more simply constructed than the Gillings' unit, and which has inherent features where are not present in the Gillings' unit. The present invention constitutes a further improvement over the unit disclosed in U.S. Pat. No. 4,431,419.